Which Chronotherm III model? The ones I've seen have the system light to the right of the word SYSTEM, not under it. Anyhow, there might have been variations, does not sound like a big deal of a difference.
So, yellow light next to SYSTEM just means that the thermostat is calling for either heating or cooling, depending on which mode it's in. Given that it's summer (here in the North Hemisphere), I imagine it's on COOL, so the AC should be ON. Does the AC run?